Lateral oscillation to form ink droplets

ABSTRACT

An electrostatic, ink-jet printing apparatus wherein electrostatically charged ink is extracted from a nozzle in an axial direction by an electrostatic potential existing between the nozzle and a valving electrode. The droplets of ink are then deflected in orthogonal, lateral directions by two pairs of deflection electrodes in order to trace an indicium on the record medium. An additional pair of deflection electrodes is placed in the region upstream of the point in which droplet formation takes place. A sinusoidal voltage is applied between these additional deflecting electrodes, shaking the stream of ink in a lateral direction in order to facilitate droplet formation and promote uniform droplet size and spacing.

United States atent 1 Sept. 5, 1972 Dunlavey [54] LATERAL OSCILLATION TOFORM INK DROPLETS [72] Inventor: Robert J. Dunlavey, Glenview, Ill.

[73] Assignee: Teletype Corporation, 2, Skokie, Ill.

[22] Filed: Aug. 20, 1970 {21 Appl.No.: 65,572

52 u.s.c1 ..346/75 [51] Int.Cl ..G01d 15/18 [58] FieldofSearch..346/1,75, 140

[56] References Cited UNITED STATES PATENTS 3,579,245 5/1971 Berry..'.....346/75X- 3,596,275 7/1971 Sweet ..346/75x 3,060,429 10/1962Winston ..346/1 3,484,794 12/1969 Winston 346/75 3,500,436 3/1970 Nordin..'..346/7 5 FOREIGN PATENTS OR APPLICATIONS Gr B it i "f.'"fjf'.ff".3 z

714,216 7/1965 can3a.;;..; ..346/75 Primary Examiner-Joseph W. l-lartaryAttorney-J. L. Landis and R. P. Miller [57] ABSTRACT An electrostatic,ink-jet printing apparatus wherein electrostatically charged ink isextracted from a nozzle in an axial direction by an electrostaticpotential existing between the noule and a valving electrode. Thedroplets of ink are then deflected in orthogonal, lateral directions bytwo pairs of deflection electrodes in order to trace an indicium on therecord medium. An additional pair of deflection electrodes is placed inthe region upstream of the point in which droplet formation takes place.A sinusoidal voltage is applied between these additional deflectingelectrodes, shaking the stream of ink in a lateral direction in order tofacilitate droplet formation and promote uniform droplet size andspacing.

1 Claim, 2 Drawing figures PATENTEMEH m2 3.689.936

INVENTOR ROBERT J. DUNLAVEY BY M ATTORNEY This invention relates toprinting apparatus andmore particularly to ink-jet printing apparatus.in which the ink is guided electrostatically onto a record medium,

- wherein varying forces are applied to the ink in order to facilitatedroplet formation.

BACKGROUND OF THE INVENTION ,U.S. Pat. No. 3,060,429 granted to OR.Winstonon Oct. 23,1962, shows an electrostatic printer in which a streamof ink is extracted froina nozzle, by the electrostatic potentialexisting between the nozzle and'a valving electrode. Two orthogonalpairs of deflection electrodes are placed downstream of the valvingelectrode and voltage differences applied between the electrodes of eachpair deflect the stream of ink in the vertical and horizontaldirections,causing the stream of ink to trace an indicium on a'record medium. Inorder to facilitate formation of droplets of uniform size and spacing,apparatus has been disclosed in U.S. Pat. No. 3,500,436 granted toRobert W. Nordin on Mar. 10, 1970, to vibrate the nozzle of anelectrostatic printing apparatus. A commonly assigned copendingapplication Ser. No. 688,947 filed on Dec. 7, 1967, by James M. Berrydiscloses applying an alternating electrostatic field between the nozzleand .the valving electrode, thereby superimposing an alternatingpotential upon the constant DC potentialthat extracts the ink from thenozzle. It has been observed that shaking the nozzle of an ink-jetprinter is not optimally effective due to the cushioning effect of thatportion of the ink jet that extends immediately in front of the nozzle.For similar reasons it has been found not to be optimally effective toapply alternating axial potentials between the valving electrode and thenozzle. f

It is an object of the present invention more effectively to cause thestream of an ink-jet printer to break into droplets of uniform size andspacing.

SUMMARY OF THE INVENTION In accordance with the present invention, astream of electrostatically charged liquid ink is formed substantiallyalong an axis and is shaken in adirection perpendicular to the axis tofacilitate the breakup of the stream into droplets of uniform size andspacing. In a preferred embodiment of the invention the stream isextracted from a nozzle in an electrostatic field existing between thenozzle and a valving electrode or grid placed some distance in front ofthe nozzle. The ink flows through an aperture in the valving electrodeand proceeds to pass between two pairs of deflection electrodes whichcause the ink to be deflected in orthogonal directions in order to tracean indicium on a record medium. The improvement comprises placing anadditional pair of deflection electrodes in the region upstream of thepoint at which droplet formation takes place and applying a varyingpotential between the two additional deflection electrodes, therebyshaking the stream of ink droplets in a direction substantiallyperpendicular to the axis of the stream of ink in order to set up(bulges) in the stream of ink to facilitate the development of equallysized droplets with approximately equal spacing between them.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be morecompletely understood from the following detailed description whenconsidered in conjunction with the accompanying drawing wherein;

. FIG. 1 is a schematic diagram of an ink-jet printer having anadditional set of deflection electrodes placed upstream of the point atplace; and

FIG. 2 is an enlarged view of the nozzle and stream inthe system shownin FIG. 1.

DETAILED DESCRIPTION Referring now to the accompanyingdrawing and moreparticularly to FIG. 1, a record medium 10 such as paper, is placed infront of a conductive platen 11 as shown in the above-mentioned Winstonpatent. A

which droplet formation takes streaml2 of ink iselectrostatically drawnfrom a nozzle 13 and is accelerated through an aperture 40 in a valvingelectrode-41 that is maintained at a high voltage withrespect to thenozzle 13.After passing through the aperture 40, the ink stream '12proceeds between a pair of vertical deflection electrodes 42 and 43 andthenthrough a pair of horizontal deflection electrodes 44 and 45. U.S.Pat. No. 3,432,844 granted to Charles R. Winston on Mar. 11, 1969,discloses a logic system for applying deflection voltages to thedeflection electrodes 42, 43, 44, and 45 in order to cause' the stream12' of ink to trace indicia on the surface of the recor medium 10. R Ithas been found that under the parameters disclosed in theabove-mentioned Winston 429 patent, the ink stream extracted from thenozzle 13 breaks into droplets approximately half-way between the end ofthe nozzle 13 and the valving electrode 41. It is a basic principle ofoperation of the deflection electrodes 42- to-45 that each individualdroplet of ink should contain substantially the same charge-to-massratio. Similarly the solidstream itself should contain the same charge-'to-mass ratio as each individual droplet.

In order to facilitate the break-up of the stream of ink into individualdroplets, two additional deflection electrodes 52 and 53 are placedacross the stream of ink. These two parallel deflection electrodes arein a plane that is substantially perpendicular to the axis defined bythe stream 12 of ink. The additional deflection electrodes 52 and 53 areplaced upstream of the location at which droplet formation takes place.

As the jet of ink issues from the nozzle 13, it rapidly acceleratesunder the influence of the field between the nozzle 13 and the electrode41. This causes the jet or stream to be reduced in diameter. When thediameter of the jet is small enough, the jet becomes unstable and beginsto bulge and neck down (forming varicosities) at more or less irregularintervals, preparatory to breaking into droplets. The length of theseintervals increases as the fluid continues to be accelerated. It is wellknown that the instability of a stream of jet of liquid is presentwhenever the circumference of the jet is less than or into the jet atthis point of the beginning of instability for overriding the tendencyto irregular droplet size and frequency of droplet formation in order tocontrol the tial keeps reversing, the stream is effectively shaken up Yand down near the point where instability begins, the stream axis beingalternated in whip-like fashion and thus determining the regularity andfrequency of the droplets and their intervals.

The excursion of the stream 12 in a direction perpendicular to its axisof flow gradually changes to axial varicosities as shown in FIG. 2. Thischange from lateral to axial oscillation might not be unlike thephenomenon of a spring-mass system having two degrees or dimensions offreedom. Oscillations in one dimension gradually change to oscillationsin the other dimension and back again. However, in the case of the fluidof FIG. 2, the jet breaks into droplets before the oscillationscanchange back again.

It will be evident that the average potential of the electrodes 52 and53 should be maintained somewhere between the voltages 'of the nozzle 13and the valving electrode 41. This can most easily be done by connectingtwo resistors 60 and 62 between the deflection electrodes 52 and 53 anda potential reference terminal 64. The terminal 64 is then maintained ata potential that is appropriate to the location of the auxiliaryelectrodes 52 and 53 within the electrostatic field between the nozzle13 and the valving electrode 41.

As one specific example of the present invention, in an apparatussimilar to that shown in FIG. of the above-mentioned Winston patent 429,auxiliary electrodes comprising two conductive wires having diameters of0.00l-inch are mounted 0.008-inch apart spanning an aperture with adiameter of 0.125-inch 7 call through a 0.0l0-inch-thick plastic board.The board is mounted 0.020-inch away from the nozzle with the wires onthe side of the board nearest the nozzle. An average potentialdifference of. 2,050 volts is maintained between the nozzle and theauxiliary electrodes with a peak-to-peak, 20 kilocycle AC potential of530 volts applied between the auxiliary electrodes.

As another example, thewires canb'e maintained at an average potentialof 2,150 volts with respect to the nozzle with a 20 kilocycle,peak-to-peak, AC potential I of 1,200 volts applied between theauxiliary electrodes.

Although a particular embodiment of the invention is shown in thedrawing and has been described in the foregoing specification, it is tobe understood that other modifications of this invention, varied to fitparticular which do not constitute. departures from the true scope ofthe invention.

Whatis claimed is: v I. In apparatus for forming a succession ofelectrium orm size an spacing and including means generating anelectrostatic field for taperingly drawing a stream of ink in acontinuous liquid phase from a nozzle substantially along the axisthereof, the improvement comprising:

a pair of electrodes, as shaking means, spaced from said nozzle anddisposed in parallel about said stream along the stream taper whereatthe circum ference of the stream is no greater than the interval betweenvaricosities that would develop in the stream in the absence of saidshaking means for producing a periodically varying electrostaticpotential field transverse to said stream and shifting the stream axisfrom the axis of the nozzle at a frequency corresponding to thefrequency of droplet formation.

charged li uid ink droplets of substantially

1. In apparatus for forming a succession of electrically charged liquidink droplets of substantially uniform size and spacing and includingmeans for generating an electrostatic field for taperingly drawing astream of ink in a continuous liquid phase from a nozzle substantiallyalong the axis thereof, the improvement comprising: a pair ofelectrodes, as shaking means, spaced from said nozzle and disposed inparallel about said stream along the stream taper whereat thecircumference of the stream is no greater than the interval betweenvaricosities that would develop in the stream in the absence of saidshaking means for producing a periodically varying electrostaticpotential field transverse to said stream and shifting the stream axisfrom the axis of the nozzle at a frequency corresponding to thefrequency of droplet formation.